Method and apparatus for aircraft pitch and thrust axes control

ABSTRACT

An aircraft automatic or semiautomtic vertical path control system which coordinates operation of pitch and engine thrust control systems to transfer speed control from one system to the other depending on a requirement to climb, descend, or maintain altitude as determined by the polarity and magnitude of the difference between a selectable desired altitude and current actual altitude.

this is a continuation of copending application Ser. No. 06/654,917filed on Sep. 27, 1984, now abandoned, which is a continuation ofapplication Ser. No. 06/419,673 filed on Sep. 20, 1982, now abandoned,which is a continuation of Ser. No. 06/099,686 filed on Dec. 3, 1979,now U.S. Pat. No. 4,357,663.

This invention relates to vertical flight path control of an aircraftand more particularly to method and apparatus for achieving throughautomatic or semiautomatic control, single mode selection for bothautopilot and autothrottle.

Improved speed control of an aircraft which is being automaticallycontrolled simultaneously in the pitch axis by elevator control and inthe longitudinal axis by throttle control is exemplified by U.S. Pat.No. 2,888,219 to Beers, et al. issued May 26, 1959. Beers, et al. isillustrative of the use of a pitch command rate input signal into thethrottle control law to automatically compensate for speed changes dueto aircraft pitch attitude changes In contrast, a preferred embodimentof the present invention relates to a means for selecting theappropriate modes of operation for the elevator and throttle systems toprovide automatic climb, descent, and altitude capture and hold througha common set of select controls without special emphasis on the specificcontrol laws or signal sources required to implement such control lawsused in the various modes of operation

An early U.S. Pat. No. 2,961,200 to Seliger, et al. issued Nov. 22, 1960relates to an automatic altitude hold and speed hold system forpropeller driven aircraft. Seliger, et al discloses the physicalimplementation (sources required) for acquiring and processing therequired signals In contrast, a preferred embodiment of the presentinvention is not concerned primarily with the signal acquisition andprocessing of control laws but with logic control of operational modesof aircraft elevator and thrust systems.

U.S. Pat. No. 2,933,268 issued April 19, 1960 to Jude, et al. shows asystem for providing stall prevention for an aircraft which is beingautomatically controlled in the pitch (elevator) and longitudinal(thrust) axis. In contrast, Jude, et al. is further directed toautomatic compensation for aircraft configuration changes (gross weight,flap position, etc.) and the effects of wind gusts.

A further exemplary reference in the field of altitude and thrustcontrol systems includes U.S. Pat. No. 3,945,593 to Schanzer issuedMarch 23, 1976. Schanzer shows automatic control of elevator andthrottles of an aircraft in a manner for reducing coupling between pitchand longitudinal axis with emphasis on control laws and aircraft statevariables to implement the control laws.

Turning now more specifically to aircraft vertical path control aspresently practiced, it should be noted that during the climb anddescent phases of a flight, an aircraft is commonly controlled in speedthrough the elevator system and in rate of climb and descent throughengine thrust. However, during the cruise phase or whenever a particularaltitude must be maintained, the altitude of the aircraft is controlledthrough the elevator and the speed is controlled through engine thrust.

Desired speeds and rates of climb or descent are variables which aredependent on many factors. For example, the climb speed may beestablished initially as an indicated airspeed (knots) for the firstportion of the climb and subsequently established as a desired machnumber; with both values being determined to provide, e.g. minimum fuelto altitude, or minimum time to altitude. Similarly, the engine thrustsetting is commonly established for maximum fuel conservation, or, forother considerations. Also, during cruise, the altitude and speed (orMach number) are variable and chosen to provide, for example, maximumrange.

When a pilot is manually flying the aircraft, he is required to controlthe aircraft elevator and engine thrust, through the manual controls, toestablish and maintain the desired values of speed, altitude, rate ofclimb, etc., as indicated to him by the various aircraft instruments.Prior automatic and semiautomatic flight control systems have so farprovided automatic, or semiautomatic modes of control in one or both ofthe control axes.

Such devices as engine thrust (EPR or N₁) control and airspeed and MachHold control modes for the elevator have been utilized, and in theutilization of these devices, the pilot selects the appropriate mode ofoperation for the pitch (elevator control) axis and for the enginecontrol.

According to present flight control practices it is possible to haveboth control systems engaged or only one control system engaged andoperate the other manually. However, apart from interlock devices whichprevent both systems from trying to control the aircraft speed at thesame time, the two control systems are independent.

Therefore, in the aforementioned systems, the pilot must manually effectthe change over from one device controlling speed to the other Forexample, the pilot may engage the thrust control system to maintain adesired engine speed (N₁) or Engine Pressure Ratio (EPR) and engage thepitch (elevator) control system to maintain the desired indicatedairspeed (IAS) or Mach number for climb. If now, a given altitude mustbe maintained, the pilot must engage the pitch axis in an altitudecontrol mode and change the thrust control to maintain aircraft speed orMach number. If then, subsequently, the pilot wishes to resume climbinghe must change the operating modes of both the pitch and thrust controlsystem to establish the desired flight path. Furthermore, these actionsoften necessitate the manipulation of speed select control devicesand/or the precise operation, in time, of speed control modes if thecontrol mode is of the hold type rather than the select type.

Accordingly, it is an object of this invention to provide a single speed(IAS or Mach) control device which provides the desired reference valuewhether the pitch axis control system or the engine thrust controlsystem is controlling the aircraft speed.

It is another object of the invention to provide a means of selectingand displaying a desired altitude and an automatic control to captureand maintain that altitude through the pitch control system.

It is a further object of the invention to provide means forautomatically capturing and maintaining the present altitude.

It is yet another object of the present invention to provide a methodfor coordinated operation of pitch and engine thrust control systems ofan aircraft to other depending upon current requirements: i.e.,climbing, descending, or maintaining altitude.

It is a still further object of the invention to provide system couplingthrough a further device containing preprogrammed or pilot programmablevertical path profiles to generate the values, either fixed or variable,for aircraft speed, engine thrust, and altitude.

It is still another object of the invention to provide means forcoupling and uncoupling the computed control outputs to either theautomatic elevator and engine control systems or to couple the pitchcontrol outputs to a Flight Director for semiautomatic operation.

It is a still further object of the invention to provide the currentmaneuver requirements, i.e. to climb, to descend, or to maintainaltitude by means of a device which determines the polarity andmagnitude of the difference between a selectable desired altitude andcurrent actual altitude.

These and other objects are achieved in accordance with a preferredsystem embodiment of this invention which includes a single speedcontrol device and readout which provides the desired reference valuewhether the pitch axis control system or the engine thrust controlsystem is controlling the aircraft speed, a means for selecting anddisplaying a desired altitude and an automatic control to capture andmaintain that altitude through the pitch control system, a means forautomatically capturing and maintaining the present altitude, acoordinated operation of the pitch and engine thrust control systems toautomatically transfer speed control from one system to the otherdepending on whether the aircraft is commanded to climb, descend, ormaintain altitude, a logic system based on the polarity and magnitudedifference between the selected altitude and present altitude fortransferring aircraft speed control automatically from the engine thrustcontrol system to the pitch control system and also automaticallyestablishing the correct engine control operation depending upon whetherthe pilot wishes to climb or descend, means for the system to beconnected to another device which contains preprogrammed or pilotprogrammable vertical path profiles to generate the values either fixedor variable for aircraft speed, engine thrust and altitude, and means tocouple and uncouple the computed control outputs to either the automaticelevator and engine control systems or to couple the pitch controloutputs to a flight director for semiautomatic operation.

A full understanding of the invention, and of its further objects andadvantages and the several unique aspects thereof, will be had from thefollowing description when taken in con]unction with the accompanyingdrawings in which:

FIG. 1 is a functional block diagram of an aircraft autopilot andautothrottle control system showing the present interactive flightcontrol mode logic and display;

FIG. 2 is a front view of the automatic flight control system modecontrol panel including controls and displays for use in an autopilotand autothrottle control system in accordance with a preferredembodiment of the present invention; and,

FIG. 3 is an enlarged fragmentary view of two parts of the automaticflight control system mode control panel of FIG. 2 including inschematic form the present interactive vertical path control systemproviding integrated pitch and thrust control.

Turning now to FIG. 1, a complete pitch axis 64 and engine controlsystem 84 is seen including autopilot 16 and autothrottle 18 utilizingstate of the art control laws Upstream from pitch axis 64 and enginecontrol system 84, mode control circuit 12 utilizes a single set ofpilot selectable controls, viz mode select, speed, and altitude select(as shown in more detail in the schematic embodiment shown in FIG. 3) tocontrol autopilot system 16 and autothrottle control system 18 of pitchaxis 64 and engine control system 84.

Turning now to automatic flight control system mode control panel 10 ofFIG. 2; it will be readily seen when comparison is made with FIG. 3,that the controls and displays of interest in control of the aircraftare mode select control 20 (shown as but not required to be, a pushbutton type control), speed select control 22, selected speed display24, (parameter) airspeed or Mach select control 26, altitude selectcontrol 28, and selected altitude display 30. In FIG. 2, one channel ofautopilot is required to be engaged in CMD (at 32) or F/D (at 34).Another standard control consideration in deployment of the presentsystem embodiment is that autothrottle 18 (shown in FIG. 1) must bearmed at 37 on FIG. 2 (or t he speed reference bug on Mach-AirspeedIndicator used for manual throttle control).

FLIGHT LEVEL CHANGE MODE (FL CH)

The FL CH mode of operation is manually selected by operating FL CH MODEREQUEST CONTROL switch 20 shown in FIG. 3. Altitude logic circuit 40operates to determine the polarity and magnitude of altitude errorsignal 42 (which is the difference at the output of combining circuitmeans 45 between aircraft altitude signal 44 provided by aircraft airdata system 46 and selected altitude signal 48 determined by flight crewmanual selection at altitude select control 28).

If altitude error signal 42 is low valued, i.e. representative of Δh inthe expression:|Δh|>xft, then AND gates 52, 56 and 58 are inhibited, sothat relays 60, 62, and 66 cannot be energized. Also, if altitude errorsignal 42 is low valued, the selection of the FL CH MODE isautomatically cancelled and mode control circuit 12 reverts to theAltitude Hold mode of operation (a feature not explicitly shown in FIG.3). The value of X is a variable dependent upon altitude capture controllaws and the current conditions e.g. at, or close to, the selectedaltitude; climbing towards the selected altitude; or descending towardsthe selected altitude.

If altitude error signal 42 is greater than X ft, either positive ornegative, AND circuit 52 is turned ON by the logic combination(|Δh|>xft) of altitude logic circuit 40 AND FL CH MODE REQUESTED,thereby causing relay 60 to be energized. Energizing relay 60 switchesthe output to pitch axis control system 64 from an altitude error (Δh)representative signal 42 to a speed error (ΔIAS or ΔM) representativesignal 47 and the pitch axis will compute output commands to control theaircraft speed to the selected value by means of elevator control Theselected speed reference can be either an airspeed value or a Machnumber value, and the elevator control can be either direct (through thepitch autopilot servo) or indirect (through manual control inputs inresponse to flight director pitch commands generated by the pitch axiscontrol system.

If the aircraft is below the selected altitude, i.e. Δh>+X, AND circuit56 will also be turned ON which causes relay 62 to be energized andallows thrust error signal 70 (provided by subtraction of signal 72representative of desired thrust from signal 74 representative of actualthrust in combining circuit 76) to be coupled downstream through thenormally open contacts of relay 62.

Desired thrust signal 72 is generated by thrust rating select controlmeans 78 and comprises a forward thrust limit value selected by the crewor alternatively, by an aircraft performance computer. Since relays 62and 66 cannot both be energized simultaneously, relay 66 will bedeenergized and thrust error signal 70 will be coupled on downstreamthrough the normally closed contacts of relay 66 to engine controlsystem 84. The engine control system responds by automatically drivingthe engine controls to capture and maintain the selected forward thrustlimit value. In this condition, the excess engine thrust will cause theaircraft to climb with the aircraft speed controlled through theelevator as hereinabove described.

The aircraft will continue to climb in this manner until it approacheswithin X ft of the selected altitude, at which point AND functioncircuits 52 and 56 are both turned off again which results in relays 60and 62 being deenergized. This action causes the output to pitch axiscontrol system 64 to revert lo altitude error signal 42 and control theaircraft to capture and maintain the selected altitude through elevatorcontrol, and the output to engine control system 84 to revert to speederror 47 and control the engine thrust to maintain selected speed 80 setby speed select control 22.

If altitude error signal 42 is of negative polarity, that is, Δh<-X whenthe FL CH mode is selected by climb/descent mode request control 20,i.e. the aircraft is above the selected altitude, a similar sequence ofevents to the preceding will occur in mode control circuit 12 exceptthat, in this case, relay 66 will be energized (by the AND function ofcircuit 58) instead of relay 62. In this condition the output to enginecontrol system 84 will be a command to a predetermined "IDLE" position.

The idle position may be a variable, depending on engine anti-ice ON/OFFetc., and may also be a "one-time" command to allow the flight crew tomake subsequent manual adjustments of the thrust setting. The reductionin thrust will cause the aircraft to descend towards the selectedaltitude with the aircraft speed being controlled to the selected valuethrough elevator control.

Similarly to the climb case, when the aircraft descends to within X feetof the selected altitude, the pitch axis control parameter will revertto altitude error causing the aircraft to capture and maintain theselected altitude through elevator control and the engine control systemcontrol parameter will revert to speed error causing the aircraft tomaintain the selected speed through thrust control.

We claim:
 1. An aircraft flight path control system comprising a pitchaxis control system (64), and engine thrust control system (84), andmeans for providing a signal representative of a desired selectedreference speed (22), means (12) for providing logic command signals inresponse to desired climb, descend or maintain altitude maneuvers ofsaid aircraft; said flight path control system further comprising aselectable flight level change mode means (FLCH) (20) for enablingswitching means (60, 62, 66) responsive to said logic command signalsfor automatically selecting said pitch axis control system (64) tocontrol the aircraft to said selected reference speed (22) when saidaircraft is commanded to climb or descend, and for automaticallyselecting said engine thrust control system (84) to control the aircraftto said selected reference speed (22) when said aircraft is commanded tomaintain a predetermined altitude.